Heise Open Source provides an extensive breakdown of the innovations present in the latest release of the Linux kernel, announced by Linus Torvalds. This version adds the first version of Ext4 as a stable filesystem, the much-anticipated GPU memory manager which will be the foundation of a renewed graphic stack, support for Ultra Wide Band (Wireless USB, UWB-IP), memory management scalability and performance improvements, a boot tracer, disk shock protection, the phonet network protocol, support of SSD discard requests, transparent proxy support, high-resolution poll()/select()... full Changelog here

'Monolithic' refers to the memory model used by the kernel. The Linux kernel, like many other common kernels (eg BSD, BeOS, Syllable), is monolithic because all kernel code including modules runs in the same memory space; ie any kernel code, whether compiled into the kernel or in a module, can access any variables or data structures of any other part of the kernel. A consequence of this is that a crash or bug in a module can corrupt the whole kernel.
The usual example of a non-monolithic kernel is Minix, http://en.wikipedia.org/wiki/MINIX.

While Linux is not a microkernel it isn't as monolithic as earlier versions of Unix. A lot has been moved outside the kernel and it is extremely modular, much more so than Windows.

One might actually say that Windows Vista is less monolithic than Linux, since the new video driver system has video drivers running in userspace. This means that a buggy video driver won't crash the whole system - it just gets safely restarted and everything goes on as usual.

Vista's graphics driver model is actually similar to Linux because there are two parts to the driver, one in kernel space for things like memory management and another part resides in userspace to handle things like GL acceleration. Linux also has split graphics drivers and DRI2 introduces a kernel memory manager for graphics much like Vista has.

It is either a microkernel or Monolithic. How much of the linux kernel is actually executed in userland?

That's simply not true. Most kernels mix elements of a microkernel with elements of a monolithic kernel. Look at OSX which has a hybrid Mach/FreeBSD kernel. Linux has things like libusb, udev, and fuse which operate from userspace.

The graphics part of the driver stack is shared with other OSes that use X.org like OpenSolaris.